Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
  • G01N21/412—Index profiling of optical fibres

Definitions

• This invention concerns the measurement of refractive index profile across an object which is approximately cylindrical, such as an optical fibre, or an optical fibre preform, the measurement being made transverse to the cylindrical axis.
• objects ideally have circular symmetry and are- invariant to the axial direction, but in practice major variations from the ideal conditions occur.
• Application of the present invent ⁇ ion allows the variations to be sensed and quantified.
• the term 'light' means electromagnetic radiation at visible, ultraviolet and infrared wavelengths.
• P.L. Chu describes a method of measuring the refractive index profile of an optical fibre preform by scanning a laser beam of very small diameter across the preform in the radial direction, i.e. transverse to the cylindrical axis of the preform, and sensing the deflection of the output beam as a function of radial position of the input beam.
• the deflection function measured in this way is numerically transformed to determine the refractive index profile.
• This method requires an input beam of very small diameter, which may be difficult to achieve, and use of a laser introduces spurious interference patterns which may be difficult to eliminate.
• OMPI W arrangement which has a precisely uniform response in this direction.
• Another difficulty is that strict validity of the theory requires the plane in which intensity is observed to be placed at a distance from the preform which is large compared to its radius; in practice the plane in which intensity is observed must be close to the preform to eliminate the effect of crossing or superimposition of beams transmitted through different sections or through opposite halves of the preform so that a single-valued output is achieved.
• the object of the present invention is to provide an improved method of sensing the deflection function of a cylind ⁇ rical object.
• a method of sensing the optical deflection function of an approximately cylindrical object comprises;
• illuminating the object over its width to be tested with a colli ⁇ iated beam of light focusing the light transmitted by the object so that in the focal plane the distance of transmitted light from the optical axis in a direction perpendicular to the cylindrical axis of the object is linearly proportional to the angle through which light has been deviated by the object; optically modulating the focused light so that a property of the light varies as a function of said distance; and receiving the modulated light in an image plane, whereby the deflection function of the object can be derived.
• the focused light may be modulated so that either a temporal or a spatial property of the light varies in a direction parallel to said direction.
• a temporal modulation the light is pulsed, and the pulse width or pulse phase varies in said direction.
• a spatial modulation the intensity or the shadow height of the modulated light varies with said ' .distance.
• apparatus for sensing the optical deflection function of an approximately cylindrical ob ⁇ ject comprising in series array optical focusing means, optical modulating means, optical receiving means, and calculating means, arranged so that when the object is illuminated by a beam of collimated light, the receiving means receives light having a modulation which varies along a direction perpendicular to the optical axis of the apparatus and to the cylindrical axis of the object, said varying modulation indicating the- angle through which light has been deviated by the object, and the calculating means calculating from said varying modulation the optical deflection function of the object.
• the transmitted light is focused by a spherical lens, and the focused light is temporally modulated by repetitive movement of a shutter in the focal plane parallel to said radial direction, the time which elapses between start of a shutter sweep and the time the shutter extinguishes light received at any position in the image plane displaced from the optical axis in a direction parallel to said radial direction varying in accordance with the deflection function,.
• the shutter may be a rotary chopper blade, or alternatively, the shutter may vibrate linearly, for example when a shutter blade is attached to a resonating tuning fork.
• a shutter is provided with repetitive movement in the focal plane perpendicular to said radial direction, the mark-space ratio of said shutter varying as a function of distance in the focal plane from the optical axis.
• the shutter will usually be a conventional rotary chopper having curved blade edges.
• the mark-space ratio of the transmitted light at any position in the image plane displaced from the optical axis in a direction parallel to said radial direction varies in accordance with the deflection function.
• the light transmitted by the object is focused by a spherical lens and the focused light is spatially modulated by a filter having a transmittance which varies in the focal plane in a direction parallel to said radial direction *
• the intensity of the " transmitted light at any point in the image plane displaced from the optical axis in a direction parallel to said radial direction provides an indication of the deflection suffered by the ray present at that point.
• the transmitted light is focused by means of a cylin ⁇ drical -Tens,-arranged ' with -its cylindrical!axis p ' arallel ' to the ⁇ cylindrical axis of" the object, arid " the focuse ' d light is spatially modulated by a " knife edge- in the focal plane, " whereby a shadowgraph is pr ⁇ duce ' d- in the image- ' ⁇ lane in which"the shadow boundary -corres- ponds--to the deflection function of the object.
• the knife edge may be straight and arranged to lie at an angle to both said orthogonal axes in the focal plane, alteration of said angle altering the magnitude of the shadowgraph co-ordinate in the direction parallel to the cylindrical axis of the object.
• the knife edge may be curved, such as "s" shaped or circular, in which case the shadowgraph will be related to the deflection function according to the known mathematical form of the knife edge.
• Figures 1 and 2 illustrate apparatus for sensing refractive index profile of an optical fibre preform using respectively temporal and spatial coding
• FIG. 1 is a ray diagram of part of Figure 1;
• Figure 4 is a ray diagram of part of Figure 2 showing the production of a shadowgraph, and
• Figures and 6 show two forms of rotary chopper for temporal modulation.
• Figure 1 is a view from above and Figure 2 is a side view of two different embodiments of the invention.
• light from an arc lamp 10 is collimated by a collimator 12 and illuminates the full diameter of an optical fibre preform 14 with a collimated beam of light 16.
• the preform 14 is supported in a transparent, parallel-sided container 18 of index-matching liquid, and the container is sealed by "0" rings 19 which allow the vertical position of the preform 14 to be altered so that different positions along the preform length can be tested * ,
• the container is optionally supported by a stepping table 17 which allows the preform to be scanned through the incident beam.
• a high-quality spherical lens 20 such as a photographic camera lens.
• a modulator 22" is--placed ' in the'-' focal plane of the lens 20, and a single photodiode 24 in the image plane of the lens can be stepped by a stepper-motor driven translation slide 26 along a horizontal axis perpendicular to the cylindrical axis of the preform 14, as indicated by the dotted line.
• the photodiode 24 and modulator 22 are connected to a Time Interval Counter 28, which, together with the translation system 26, is in turn connected to a microprocessor 29 which supplies a display unit 30.
• a ra y of light entering the preform 14 at position _ from the optical axis is deviated by an angle 0 as shown, and, if the transmitted light is viewed in a plane normal to the incident beam, the intensity distribution in the direction perpendicular to the cylindrical axis of the preform is related to the radial refractive index profile of the preform.
• a lens 20 is placed in the transmitted beam, then in the focal plane of the lens (sometimes known as the Fourier transform plane) the linear distance ⁇ of any beam from the optical axis is proportional to the angle of incidence of the beam on the lens, i.e. to the angular deviation_0 of the beam, provided the angle is small.
• the relationship is given by:-
• the fixed time-reference is provided by a static photodiode and light source fixed to the body of the chopper at position 22 as is conventional for the provision of a reference signal in light-chopping applications
• the time reference is used t to provide a START signal to the Time Interval Counter ' 28, corresponding to a known position of the chopper blade in space, and termination of illumination of the photodiode 24 provides a STOP signal to define pulse length for each value of y_'.
• the microprocessor then calculates the angle_ ⁇ for each y and computes the radial refractive index distribution n(r) of the preform from the deflection function 0 (y) by application of the transform;
• n(r) .n. fi i --i ⁇ ii , 2,- - (y) (y • -r )- dy (2)
• n n(a)
• the index of the index matching fluid r is the o — radial co-ordinate
• a_ is the radial co-ordinate of the scan starting point and must be larger than the radius of the preform
• _0 is related to 0 by Snell's law, i.e.
• the index profile n(r) is displayed on the display unit 30.
• - 8 The experimental configuration for temporal coding utilising a sweep orthogonal to that described above, i.e. in a direction parallel to the preform axis, is similar to that shown in Figure 1 but omits the Time Interval Counter 28 and does not require a reference signal.
• the modulator 22 is a rotary chopper blade chosen to have a mark-rspace ratio which varies with radial position and the chopper axis of rotation is arranged such that a different mark-space ratio pertains for each ray position oo in Figure 1.
• the associated deflection angle 0 can be found by observation of the signal mark-space ratio, normally measured by applying a low-pass filter and obtaining the average value.
• Chopper blades which have a radial mark-space ratio variation may be constructed with straight edges which do not pass through the centre of rotation i.e. non-radial edges, or with curved blades such as given by sections of a linear spiral as shown in Figure 6, " The latter gives a convenient linear variation of mark-space ratio with radial positions.
• OMPI intensity depending on the distance ⁇ from the optic axis at which they traverse the focal plane. Measurement by the photocell 24 of the intensity at position ' in the image plane permits the relationship between position y of a ray impinging on the preform and its associated deflection angle_0 to be determined.
• the microprocessor 29 relates the intensity to the deflection angle and computes the index profile using the transform given in equation (2). This arrangement is susceptible to fluctuations in the intensity of illumination.
• the effect of the cylindrical lens is to image the ray at a distance ⁇ from the central axis in the focal or Fourier plane of the lens.
• the condition for transmission is: v ⁇ an ⁇ ' (3)
• x 1 is proportional to 0_ and the shadow boundary x' (y 1 ) has the geometrical form of the deflection function of the preform.
• the shadow boundary which is the deflection function
• a diode array reference 3 in Figure 2
• the outputs of the diodes are processed to-rev ⁇ al,the geometrical /.co-ordinates of the shadow edge, and therefoce the " deflection function,, and the microprocessor - 29 calculates the radial refractive index distribution n(r) of the preform from the deflection function by application of the transform given in equation 2.
• the invention can also be applied to an optical fibre, since the distribution of angles in the lens focal-plane is similar to
• OMPI OMPI . VVIPO that of the parent preform.
• the temporal or spatial filter used to encode the fibre can be similar to that used for the preform. It will, however, be necessary to provide additional optical magnifying means so that the size of the image is sufficiently large for accurate measurement to be possible.
• the invention can be applied to an object which intentionally does not have circular symmetry.
• the refractive index profile can be determined along a plurality of different radii centred on the same point. A three dimensional profile of the object can then be constructed.
• the illuminating beam can be angularly swept, or the knife edge in Figure 2 can be scanned.
• the resolution of the system depends on the quality of the ' lens used, and, as stated above, the lens must have a numerical aperture sufficient to accept a ray of light with the largest deflection imposed by the test object.

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• Physics & Mathematics (AREA)
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• Length Measuring Devices By Optical Means (AREA)

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• 1981
  • 1981-03-11 US US06/308,542 patent/US4515475A/en not_active Expired - Lifetime
  • 1981-03-11 WO PCT/GB1981/000040 patent/WO1981002634A1/en active IP Right Grant
  • 1981-03-11 EP EP81900561A patent/EP0047272B1/de not_active Expired
  • 1981-03-11 JP JP56500782A patent/JPH0463330B2/ja not_active Expired
• 1991
  • 1991-10-25 JP JP3280047A patent/JPH0648229B2/ja not_active Expired - Lifetime

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